Start setting dso_local in clang.

This starts adding dso_local to clang.

The hope is to eventually have TargetMachine::shouldAssumeDsoLocal go
away. My objective for now is to move enough of it to clang to remove
the need for the TargetMachine one to handle PIE copy relocations and
-fno-plt. With that it should then be easy to implement a
-fno-copy-reloc in clang.

This patch just adds the cases where we assume a symbol to be local
based on the file being compiled for an executable or a shared
library.

llvm-svn: 324107